Imaging apparatus and camera

ABSTRACT

An imaging apparatus including a pixel, a current source, and a signal processing circuit. The pixel outputs signal charge, obtained by imaging, as a pixel signal. The current source is connected to a transmission path for the pixel signal and has a variable current. The signal processing circuit performs signal processing on a signal depending on an output signal to the transmission path and performs control so that a current of the current source is changed in accordance with the result of signal processing.

RELATED APPLICATION DATA

This application is a continuation of U.S. patent application Ser. No.13/155,784, filed Jun. 8, 2011, which is a division of U.S. patentapplication Ser. No. 11/952,315, filed Dec. 7, 2007, the disclosures ofwhich are incorporated herein by reference in their entirety. Thepresent application claims the benefit of priority to Japanese PatentApplication No. JP 2006-339415 filed in the Japanese Patent Office onDec. 18, 2006, the divisional of which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to an imaging apparatus havingcolumn-parallel analog-to-digital converters and a camera including theimaging apparatus.

In many cases, typical solid-state imaging apparatuses, each havingcolumn-parallel analog-to-digital converters, include counter-rampanalog-to-digital converters utilizing single-slope integration.

FIG. 1 illustrates the structure of an imaging apparatus havingcounter-ramp analog-to-digital converters utilizing single-slopeintegration.

FIG. 2 is a timing chart explaining the operation of the apparatus inFIG. 1.

The operation of the apparatus in FIG. 1 will now be described inrelation to the timing chart of FIG. 2.

Unit pixels 1 for photoelectrically converting external light intoelectrical signals are arranged in a matrix to constitute a pixel array2. It is assumed that a vertical selection circuit 3 selects a certainrow in the pixel array 2.

A pixel in the selected row outputs a pixel signal (hereinafter, a pixeloutput signal S1) through a vertical signal line vs1 extending along thecolumns of the pixel array 2. The vertical signal line vs1 is connectedto one input terminal (positive terminal) of a comparator 4 disposed foreach column. The other terminal (negative terminal) of the comparator 4receives an output signal nslope from a digital-to-analog (D/A)converter 5, the output signal being an analog signal obtained inresponse to a clock signal clk.

It is assumed that when the pixel output signal S1 transmitted throughthe vertical signal line vs1 is lower in level than the output signalnslope of the D/A converter 5, an output signal ncompout of thecomparator 4 becomes low (“L”), whereas when the pixel output signal S1of the vertical signal line vs1 is higher in level than the outputsignal nslope of the D/A converter 5, the output signal ncompout of thecomparator 4 becomes high (“H”).

The output signal ncompout of the comparator 4 is supplied to an n-bitcounter 6. The n-bit counter 6 increases or decreases a count value inresponse to the clock signal clk. FIG. 2 shows a case where the countvalue decreases by one.

The n-bit counter 6 resets its count value to an initial value i_(init)in response to a reset signal rst. When the output signal ncompout ofthe comparator 4 becomes high, the n-bit counter 6 stops increasing ordecreasing the count value and holds the count value (denoted by “i” inFIG. 2).

This count value is output as n-bit output data [n−1:0] from the n-bitcounter 6. The output of the n-bit counter 6 and outputs of othercounters in other columns are sequentially output.

SUMMARY OF THE INVENTION

In the counter-ramp analog-to-digital converters utilizing single-slopeintegration as shown in FIG. 1, disadvantageously, conversion time or aclock frequency has to be increased in order to increase the accuracy ofanalog-to-digital conversion.

For example, the number of clocks required for A/D conversion of n bitsis 2^(n)−1 cycles. When n bits are extended to n+1 bits, the number ofclocks required is 2^(n+1)−1 cycles, which is substantially twice the2^(n)−1 cycles.

To achieve the increase in the number of clocks required, the conversiontime or the clock frequency has to be increased by a factor ofapproximately two. Disadvantageously, increasing the conversion timeprevents high speed operation for realizing a high frame rate orprocessing an increased number of pixels. Unfortunately, increasing theclock frequency results in an increase in power consumption or requiresan improvement of the accuracy of elements.

Furthermore, in the counter-ramp analog-to-digital converters utilizingsingle-slope integration, disadvantageously, a variation in thresholdlevel Vth of a comparator or the accuracy of voltage output from adigital-to-analog converter affects the accuracy of theanalog-to-digital converter.

In other words, assuming that a pixel output signal has an amplitude of1 V, serving as the difference between dark and light levels, anaccuracy of 1/(2^(n)−1) V is required for analog-to-digital conversionof n bits.

For example, when n=14 (bits), 1/(2¹⁴−1)=61 μV. Since a variation inthreshold Vth of a comparator is generally on the order of severalmillivolts, it is difficult to realize high-accuracy analog-to-digitalconversion.

It is desirable to provide a high-resolution high-accuracy imagingapparatus capable of achieving high speed operation for realizing a highframe rate or processing an increased number of pixels without causingan increase in power consumption and requiring an improvement of theaccuracy of elements.

According to a first embodiment of the present invention, an imagingapparatus includes the following elements. A pixel outputs signalcharge, obtained by imaging, as a pixel signal. A current source havinga variable current is connected to a transmission path for the pixelsignal. A signal processing circuit performs signal processing on asignal depending on an output signal to the transmission path andperforms control so that a current of the current source is changed inaccordance with the result of signal processing.

Preferably, the pixel includes an amplifier transistor constituting asource follower that outputs signal charge as a pixel signal to thetransmission path. The current source includes a plurality of loadtransistors and a plurality of switches. The gates of the loadtransistors are biased using a predetermined bias signal and the sourcesthereof are connected in parallel to a predetermined potential. Eachswitch is connected between the drain of the corresponding loadtransistor and the transmission path. The signal processing circuitselectively turns on the switches in the current source in accordancewith the result of processing on the pixel signal transmitted throughthe transmission path.

Preferably, the pixel includes an amplifier transistor constituting asource follower that outputs signal charge as a pixel signal to thetransmission path. The current source includes a load transistor and aplurality of switches. The source of the load transistor is connected toa predetermined potential. The switches are connected to the gate of theload transistor and are connected to different bias-signal supply lines,respectively. The signal processing circuit selectively turns on theswitches in the current source in accordance with the result ofprocessing on the pixel signal transmitted through the transmissionpath.

Preferably, the signal processing circuit includes an integrator thatintegrates a pixel signal transmitted through the transmission path, aquantizer that quantizes an output signal of the integrator, and aselector that changes a current of the current source in accordance withthe result of quantization by the quantizer.

Preferably, the signal processing circuit includes an integrator thatintegrates a pixel signal transmitted through the transmission path, aquantizer that quantizes an output signal of the integrator, and aselector that selectively turns on the switches in the current source inaccordance with the result of quantization by the quantizer.

Preferably, the quantizer compares the level of an integrated signalobtained by the integrator with a predetermined threshold level andoutputs the result of comparison to the selector. The selector changes acurrent of the current source so that when the level of the integratedsignal is higher than the threshold level, the level of the transmissionpath is lowered, and when the level of the integrated signal is lowerthan the threshold level, the level of the transmission path is raised.

Preferably, the quantizer compares the level of an integrated signalobtained by the integrator with a predetermined threshold level andoutputs the result of comparison to the selector. The selectorselectively turns on the switches in the current source so that when thelevel of the integrated signal is higher than the threshold level, thelevel of the transmission path is lowered, and when the level of theintegrated signal is lower than the threshold level, the level of thetransmission path is raised.

Preferably, the transmission path includes a pixel output signal linethat is connected to the source of the amplifier transistor in the pixeland is connected to the current source.

Preferably, the transmission path includes a pixel output signal lineand a source follower transistor. The pixel output signal line isconnected to the source of the amplifier transistor in the pixel. Thegate of the source follower transistor is connected the pixel outputsignal line and the source thereof is connected to the current source.

According to a second embodiment of the present invention, an imagingapparatus includes the following elements. A pixel array includes aplurality of pixels arranged in a matrix, each pixel outputting signalcharge, obtained by imaging, as a pixel signal. A plurality of pixelsignal processing units are arranged for respective columns of the pixelarray. Each pixel signal processing unit includes a current source and asignal processing circuit. The current source is connected to atransmission path for the pixel signal and has a variable current. Thesignal processing circuit performs signal processing on a signaldepending on an output signal to the transmission path and performscontrol so that a current of the current source is changed in accordancewith the result of signal processing.

According to a third embodiment of the present invention, a cameraincludes an imaging apparatus and an optical system that forms a subjectimage on the imaging apparatus. The imaging apparatus includes thefollowing elements. A pixel array includes a plurality of pixelsarranged in a matrix, each pixel outputting signal charge, obtained byimaging, as a pixel signal. A plurality of pixel signal processing unitsare arranged for respective columns of the pixel array. Each pixelsignal processing unit includes a current source and a signal processingcircuit. The current source is connected to a transmission path for thepixel signal and has a variable current. The signal processing circuitperforms signal processing on a signal depending on an output signal tothe transmission path and performs control so that a current of thecurrent source is changed in accordance with the result of signalprocessing.

According to the embodiments of the present invention, the signalprocessing circuit performs predetermined processing on a pixel signal,serving as signal charge obtained by imaging.

The level of the transmission path is controlled so that a current ofthe current source is changed in accordance with the result of signalprocessing.

According to the embodiments of the present invention, a high-resolutionhigh-accuracy imaging apparatus capable of achieving high speedoperation for realizing a high frame rate or processing an increasednumber of pixels without causing an increase in power consumption andrequiring an improvement of the accuracy of elements and a cameraincluding the imaging apparatus can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the structure of an imaging apparatushaving counter-ramp analog-to-digital, converters utilizing single-slopeintegration;

FIG. 2 is a timing chart explaining the operation of the apparatus inFIG. 1;

FIG. 3 is a block diagram illustrating the schematic structure of animaging apparatus according to an embodiment of the present invention;

FIG. 4 is a diagram showing the structure of a unit pixel and that of apixel signal processing unit according to the embodiment of the presentinvention;

FIG. 5 is a block diagram illustrating the basic structure of adelta-sigma (ΔΣ) analog-to-digital converter;

FIG. 6 is a diagram showing the structure of a pixel signal processingunit according to a modification of the embodiment of the presentinvention;

FIG. 7 is a diagram illustrating the structure of a unit pixel and thatof a pixel signal processing unit according to a first concrete exampleof the embodiment of the present invention;

FIG. 8 is a diagram showing the relationship between outputs of aquantizer and set levels of selection control signals in a selector of aΔΣ modulator in accordance with the embodiment;

FIG. 9 is a timing chart explaining the operation of the pixel signalprocessing unit in FIG. 7;

FIG. 10 is a diagram illustrating the structure of a unit pixel and thatof a pixel signal processing unit according to a second concrete exampleof the embodiment of the present invention; and

FIG. 11 is a diagram illustrating the structure of a camera systemincluding the imaging apparatus according to the embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will now be described withreference to the drawings.

FIG. 3 is a block diagram illustrating the schematic structure of animaging apparatus according to the present embodiment.

FIG. 4 is a diagram illustrating the structure of a unit pixel and thatof a pixel signal processing unit according to the present embodiment.

The imaging apparatus 10 includes a pixel array 12, a vertical selectioncircuit 13, and a plurality of pixel signal processing units 14 (14-1,14-2, . . . ). The pixel array 12 is composed of a plurality of unitpixels 11 arranged in a matrix. Each unit pixel 11 photoelectricallyconverts external light into an electrical signal. The verticalselection circuit 13 selects a predetermined row of the pixel array 12and drives a plurality of drive lines DRVL connected to the unit pixels.Each pixel signal processing unit 14 is connected directly (or through asource follower transistor) to a pixel output signal line VSL,constituting a transmission path arranged for each column of the pixelarray 12.

Each unit pixel 11 includes a photodiode 111, a transfer transistor 112,an amplifier transistor 113, a reset transistor 114, and a selectiontransistor 115.

The photodiode 111 converts incident light into signal charge (forexample, electrons), whose amount corresponds to the amount of theincident light, and stores the signal charge.

The source of the transfer transistor 112 connected to the cathode ofthe photodiode 111, the drain thereof is connected to a floating nodeND111, and the gate thereof is connected to a transfer selection lineTRL through which a transfer signal TR is transmitted. The transfertransistor 112 has a function of transferring the signal charge storedin the photodiode 111 to the floating node ND111 when conducting(turning on).

The source of the amplifier transistor 113 is connected to the pixeloutput signal line VSL, the drain thereof is connected to the source ofthe selection transistor 115, and the gate thereof is connected to thefloating node ND111. The drain of the selection transistor 115 isconnected to a power supply voltage source VDD and the gate thereof isconnected to a selection line SELL.

The amplifier transistor 113 has a function of outputting the potentialof the floating node ND111 to the pixel output signal line VSL when thevertical selection circuit 13 supplies a high-level selection signal SELto the selection line SELL to turn on the selection transistor 115.

The drain of the reset transistor 114 is connected to the power supplyvoltage source VDD, the source thereof is connected to the floating nodeND111, and the gate thereof is connected to a reset line RSTL. When thevertical selection circuit 13 supplies a high-level reset signal RST tothe reset line RSTL, the reset transistor 114 turns on, thus resettingthe potential of the floating node ND111.

The unit pixels 11 with the above-described structure are arranged in amatrix to form a pixel area (imaging area). The transfer selection lineTRL, the selection line SELL, and the reset line RSTL are arranged foreach row of the pixel array 12.

The vertical selection circuit 13 drives the transfer selection linesTRL, the selection lines SELL, and the reset lines RSTL.

Each pixel signal processing unit 14 includes a current source 141 and asignal processing circuit 142. The current source 141 and the signalprocessing circuit 142 are connected to the corresponding verticalsignal line VSL extending from the pixel array 12. Referring to FIG. 4,each pixel signal processing unit 14 further includes n (n is a positiveinteger of two or more) load MOS transistors LT1 to LTn and n switchingtransistors SW1 to SWn. At least one of the load MOS transistors LT1 toLTn is selectively connected to the amplifier transistor 113 in the unitpixel 11 through the pixel output signal line VSL and the correspondingswitching transistor. The amount of drive current (hereinafter, thedrive current amount) of the connected load MOS transistor is controlledin accordance with an output obtained by signal processing through thesignal processing circuit 142 (quantizer).

In the current source 141, the gate of each of the load MOS transistorsLT1 to LTn is connected to a bias-signal supply line VBIASL throughwhich a bias signal VBIAS is supplied. The load MOS transistors LT1 toLTn each function as a constant current source.

The source of the load MOS transistor LT1 is connected to a referencepotential (ground potential GND in the present embodiment) and the drainthereof is connected to the source of the MOS switch (MOS transistor)SW1. The drain of the MOS switch SW1 is connected to the pixel outputsignal line VSL.

Similarly, the source of the load MOS transistor LTn is connected to thereference potential (the ground potential GND in the present embodiment)and the drain thereof is connected to the source of the MOS switch (MOStransistor) SWn. The drain of the MOS switch SWn is connected to thepixel output signal line VSL.

The gates of the respective MOS switches SW1 to SWn are connected to acontrol output of the signal processing circuit 142.

In the imaging apparatus 10 according to the present embodiment, eachpixel signal processing unit 14 feeds a digital value, serving as anoutput of the signal processing circuit 142, back to the current source141 every pixel signal to realize a function equivalent to that of a ΔΣanalog-to-digital converter capable of achieving high-accuracyanalog-to-digital conversion.

Specifically, in column-parallel ΔΣ analog-to-digital convertersprovided for the solid-state imaging apparatus, the feedback of theoutput of each quantizer is realized by changing a current of the loadMOS transistor, constituting a source follower connected to the pixeloutput signal line. Consequently, the solid-state imaging apparatus withhigh resolution and high accuracy is realized.

The basic structure of the ΔΣ analog-to-digital converter and thecharacteristic structure of the pixel signal processing unit 14according to the present embodiment will now be described in more detailbelow.

FIG. 5 is a block diagram illustrating the basic structure of a ΔΣanalog-to-digital converter.

Referring to FIG. 5, the ΔΣ analog-to-digital converter 200 includes asubtractor 201, an integrator 202, a quantizer 203, a digital-to-analogconverter 204, and a decimation filter 205.

The integrator 202 functions as a low-pass filter (LPF). The quantizer203 has a function of adding quantization noise (i.e., a function ofdifferentiating to determine whether a value is set to “1” or “0”). Thedigital-to-analog converter 204 has a differentiating function.

In applications other than those to solid-state imaging apparatuses,e.g., in audio applications and applications to measuring instruments,ΔΣ analog-to-digital converters are used as analog-to-digital convertersfor achieving high accuracy in many cases. A conversion accuracyexceeding 16 bits is achieved.

The ΔΣ analog-to-digital converter 200 fundamentally includes theintegrator 202 and the quantizer 203 as shown in FIG. 5. The subtractor201 subtracts a feedback signal, obtained by digital-to-analogconverting an output of the quantizer 203 through the digital-to-analogconverter 204, from an analog input signal. After that, the integrator202 integrates a signal output from the subtractor 201, the quantizer203 quantizes a signal output from the integrator 202, and thedecimation filter 205 decimates a signal output from the quantizer 203,thereby obtaining a digital output signal.

The ΔΣ analog-to-digital converter 200 can achieve high-accuracyanalog-to-digital conversion. To apply the high-accuracyanalog-to-digital conversion to a solid-state imaging apparatus, adigital value has to be fed back for each pixel signal. It is difficultto realize the feedback.

In the present embodiment, the above-described feedback is realized byarranging the constant current sources including the MOS transistors(n-channel MOS transistors in the present embodiment), which areconnected to the amplifier transistor in each unit pixel 11 toconstitute a source follower circuit, and changing a current of theconstant current sources in accordance with an output of the sourcefollower circuit or an output of the signal processing circuit 142arranged in the subsequent stage, the output of the circuit 142 beingbased on the output of the source follower circuit.

Again referring to FIG. 4, as described above, the pixel signalprocessing unit 14 is connected to the pixel output signal line VSLextending from the pixel such that the load MOS transistors LT1 to LTnare connected to the amplifier transistor 113 in the unit pixel 11 viathe pixel output signal line VSL through the switching transistors (MOSswitches) SW1 to SWn, respectively. The drive current amount of the loadMOS transistors is controlled in accordance with an output of thequantizer in the signal processing circuit 142.

Specifically, in the pixel signal processing unit 14 in FIG. 4, each ofthe MOS switches SW1 to SWn is disposed between the corresponding one ofthe load MOS transistors LT1 to LTn and the pixel output signal lineVSL. Turning on and off the MOS switches SW1 to SWn changes the numberof the load MOS transistors LT connected to the pixel output signal lineVSL.

A change in voltage of the pixel output signal line VSL according to theload MOS transistors LT is expressed by the following Expression 1. Whenlet I_(load) be the drive current amount of the load MOS transistors LT,let V_(PIXEL) be a pixel signal voltage, and let V_(TH) be a thresholdlevel of the amplifier transistor 113, the current drive amount I_(load)changes depending on a voltage V_(PIXEL)−V_(TH). Consequently, an outputof the quantizer can be fed back.

$\begin{matrix}{V_{VSL} = {V_{PIXEL} - V_{{TH},{amp}} - \sqrt{\frac{2I_{load}}{\mu_{0}C_{ox}\frac{W}{L}}}}} & {{Expression}\mspace{14mu} 1}\end{matrix}$

where μ denotes the mobility of carrier, C_(ox) denotes the gatecapacitance per unit area, W denotes the gate width, and L denotes thegate width.

FIG. 6 illustrates the structure of a pixel signal processing unitaccording to a modification of the present embodiment of the presentinvention.

The difference between the pixel signal processing unit 14 in FIG. 4 anda pixel signal processing unit 14A in FIG. 6 is as follows: A currentsource 141A includes a single load MOS transistor LT1. A first MOSswitch SW1 is connected between the gate of the load MOS transistor LT1and a first bias-signal supply line VBIASL1. An nth MOS switch SWn isconnected between the gate of the load MOS transistor LT1 and an nthbias-signal supply line VBIASLn. The turn-on and -off of the MOSswitches SW1 to SWn are controlled using an output of the signalprocessing circuit 142.

Specifically, the pixel signal processing unit 14A in FIG. 6 selects anyof a plurality of bias voltages as the gate voltage of the load MOStransistor LT1 in accordance with the output of the signal processingcircuit 142. Any of the MOS switches (transistors) is selectivelyconnected to exclusively select any of the bias voltages, thus changingthe bias voltage. Consequently, the drive current amount of the load MOStransistor LT1 can be changed.

As described above, in the imaging apparatus 10 having the pixel signalprocessing unit 14 in FIG. 4 or 14A in FIG. 6, the drive current amountof at least one load MOS transistor LT is changed in accordance with apixel output of the imaging apparatus or a pixel signal obtained bysignal processing on the pixel output, so that an offset amount suitablefor signal processing on the subsequent stage can be added to the pixeloutput.

Adding the offset can perform feedback such that the source followercircuit, composed of the amplifier transistor 113 in each unit pixel 11and the load MOS transistor LT, can be used in a linear characteristicregion.

Changing the offset can cancel out a variation caused by a manufacturingprocess, e.g., a variation in threshold voltage of the amplifiertransistor 113 in each unit pixel 11.

According to the embodiments of the present invention, a high-resolutionhigh-accuracy imaging apparatus capable of achieving high speedoperation for realizing a high frame rate or processing an increasednumber of pixels without causing an increase in power consumption andrequiring an improvement of the accuracy of elements can be realized.

A more concrete example of the structure of a pixel signal processingunit will now be described below.

FIG. 7 illustrates of the structure of a unit pixel and that of a pixelsignal processing unit according to a first concrete example inaccordance with the present embodiment of the present invention.

Referring to FIG. 7, the structure of the unit pixel 11 is similar tothose shown in FIGS. 4 and 6. The structure of a pixel signal processingunit 14B is made more concrete than those in FIGS. 4 and 6.

Specifically, referring to FIG. 7, a signal processing circuit includesa ΔΣ modulator 142B that includes an integrator 1421, a quantizer 1422,and a selector 1423.

The structure of a current source 141B is similar to that in FIG. 4. InFIG. 7, n=3. The current source 141B includes three load MOS transistorsLT1 to LT3 and three MOS switches SW1 to SW3.

In the current source 141B, the gates of the load MOS transistors LT1 toLT3 are connected to the bias-signal supply line VBIASL through whichthe bias signal VBIAS is supplied. The load MOS transistors LT1 to LT3each function as a constant current source.

The source of the load MOS transistor LT1 is connected to the referencepotential (the ground potential GND in this embodiment) and the drainthereof is connected to the source of the MOS switch (MOS transistor)SW1. The drain of the MOS switch SW1 is connected to the pixel outputsignal line VSL.

Similarly, the source of the load MOS transistor LT2 is connected to thereference potential (the ground potential GND in the present embodiment)and the drain thereof is connected to the source of the MOS switch (MOStransistor) SW2. The drain of the MOS switch SW2 is connected to thepixel output signal line VSL.

Similarly, the source of the load MOS transistor LT3 is connected to thereference potential (the ground potential GND in the present embodiment)and the drain thereof is connected to the source of the MOS switch (MOStransistor) SW3. The drain of the MOS switch SW3 is connected to thepixel output signal line VSL.

The gate of the MOS switch SW1 is connected to a supply line for aselection control signal ISEL1 in the ΔΣ modulator 142B. The gate of theMOS switch SW2 is connected to a supply line for a selection controlsignal ISEL2 in the ΔΣ modulator 142B. The gate of the MOS switch SW3 isconnected to a supply line for a selection control signal ISEL3 in theΔΣ modulator 142B.

In the ΔΣ modulator 142B, the integrator 1421, the quantizer 1422, andthe selector 1423 start operating in response to a high-level selectionsignal HSEL and operate synchronously with a clock signal CLK.

The integrator 1421 functions as an LPF and outputs a signal SOUTindicating the result of integration to the quantizer 1422.

The quantizer 1422 quantizes the signal SOUT output from the integrator1421 using a predetermined threshold level VTHL and outputs a signalHSIG indicating the result of quantization to the selector 1423 and adecimation filter (not shown) in the subsequent stage.

When the level of the signal SOUT is higher than the threshold levelVTHL, the quantizer 1422 sets the output signal HSIG to a value of “1”.When the level of the signal SOUT is lower than the threshold levelVTHL, the quantizer 1422 sets the output signal HSIG to a value of “0”.

The selector 1423 sets the level of each of the selection controlsignals ISEL1 to ISEL3 in accordance with the value of the output signalHSIG of the quantizer 1422, the signals ISEL1 to ISEL3 being used forcontrol of turn-on/off of the MOS switches SW1 to SW3 in the currentsource 141B.

FIG. 8 is a diagram showing the relationship between the outputs of thequantizer 1422 and set levels of the selection control signals in theselector 1423 in the ΔΣ modulator 142B according to the presentembodiment.

In the present embodiment, in an initial state, the selector 1423 setsthe two selection control signals ISEL1 and ISEL2 to a high level (“H”)and sets the selection control signal ISEL3 to a low level (“L”) asshown in FIG. 8.

In the initial state, the MOS switches SW1 and SW2 in the current source141B turn on and the MOS switch SW3 turns off. Accordingly, the load MOStransistors LT1 and LT2 are connected in parallel to the pixel outputsignal line VSL, so that the two current sources are connected to theamplifier transistor 113 in the unit pixel to constitute a sourcefollower. In other words, in the initial state, the selector 1423selects and controls the load MOS transistors so as to obtain the drivecurrent amount of two transistors.

Referring to FIG. 8, when the output of the quantizer 1422 indicates“1”, the selector 1423 sets the three selection control signal ISEL1 toISEL3 to the high level (“H”).

When the output of the quantizer 1422 indicates “1”, the MOS switchesSW1 to SW3 turn on in the current source 141B. Accordingly, the load MOStransistors LT1 to LT3 are connected in parallel to the pixel outputsignal line VSL, so that the three current sources are connected to theamplifier transistor 113 in the unit pixel to constitute a sourcefollower. In other words, when the output of the quantizer 1422indicates “1”, the selector 1423 selects and controls the load MOStransistors so as to obtain the drive current amount of threetransistors.

Referring to FIG. 8, when the output of the quantizer 1422 indicates“0”, the selector 1423 sets the selection control signal ISEL1 to thehigh level (“H”) and sets the two selection control signals ISEL2 andSEL3 to the low level (“L”).

When the output of the quantizer 1422 indicates “0”, the MOS switch SW1turns on and the MOS switches SW2 and SW3 turn off in the current source141B. Accordingly, the load MOS transistor LT1 is connected in parallelto the pixel output signal line VSL, so that the single current sourceis connected to the amplifier transistor 113 in the unit pixel toconstitute a source follower. In other words, when the output of thequantizer 1422 indicates “0”, the selector 1423 selects and controls theload MOS transistors so as to obtain the drive current amount of onetransistor.

In the pixel signal processing unit 14B with the above-describedstructure, an output from the pixel output signal line VSL is suppliedto the integrator 1421 constituting the ΔΣ modulator 142B. The signalSOUT output from the integrator 1421 is supplied to the quantizer 1422.A pixel output signal obtained by quantization through the quantizer1422 is supplied as the signal HSIG to the decimation filter in thesubsequent stage and is also fed back to the selector 1423 to controlthe drive current amount of the load MOS transistors. This arrangementachieves a ΔΣ analog-to-digital converter.

FIG. 10 is a timing chart explaining the operation of the pixel signalprocessing unit 14B in FIG. 7.

In the initial state before the selection signal HSEL becomes high, theselector 1423 sets the two selection control signals ISEL1 and ISEL2 tothe high level (“H”) and sets the selection control signal ISEL1 to thelow level (“L”). Consequently, the MOS switches SW1 and SW2 turn on andthe MOS switch SW3 turns off in the current source 141B. In other words,in the initial state, the selector 1423 selects and controls the loadMOS transistors so as to obtain the drive current amount of twotransistors.

In this instance, when the selection signal HSEL becomes high, theintegrator 1421, the quantizer 1422, and the selector 1423 startoperating. The output signal HSIG of the quantizer 1422 varies inresponse to the output signal SOUT of the integrator 1421 (a dashed lineat “SOUT” in FIG. 9 denotes the threshold level VTHL of the quantizer1422).

When receiving the signal HSIG indicating, for example, “1” from thequantizer 1422, the selector 1423 sets the three selection controlsignals ISEL1 to ISEL3 to the high level “H” in the next clock cycle tolower the level of the pixel output signal line VSL. Consequently, theMOS switches SW1 to SW3 turn on in the current source 141B. In otherwords, when the output of the quantizer 1422 indicates “1”, the selector1423 selects and controls the load MOS transistors so as to obtain thedrive current amount of three transistors.

On the other hand, when the output signal HSIG of the quantizer 1422indicates “0”, the selector 1423 sets only the selection control signalISEL1 to the high level (“H”) so as to raise the level of the pixeloutput signal line VSL. Consequently, only the MOS switch SW1 turns onin the current source 141B. In other words, when the output of thequantizer 1422 indicates “0”, the selector 1423 selects and controls theload MOS transistors so as to obtain the drive current amount of onetransistor.

The above-described feedback achieves the ΔΣ analog-to-digitalconverter.

As described above, the arrangement in FIG. 7 can achieve a ΔΣanalog-to-digital converter. Advantageously, a solid-state imagingapparatus having column-parallel high-accuracy analog-to-digitalconverters can be realized.

In the example of FIG. 7, the first-order integrator and the 1-bitquantizer are used. The number of selectors and the number of load MOStransistors to be controlled may be changed in accordance with the orderof the integrator and the number of bits of the quantizer. Theintegrator may have either switched-capacitor circuitry or RC circuitry.

FIG. 10 illustrates of the structure of a unit pixel and that of a pixelsignal processing unit according to a second concrete example inaccordance with the present embodiment of the present invention.

The difference between the first and second examples in FIGS. 7 and 10is that the output of the pixel output signal line VSL is temporarilysupplied to a source follower 15S including a MOS transistor 15, servingas part of another transmission path, and the source follower 15S isconnected to the current source 141B so that the drive current amount ofload MOS transistors flowing to the MOS transistor 15 is changed.

Specifically, the source of the MOS transistor 15 is connected to eachof the drains of MOS transistors, constituting the MOS switches SW1 toSW3 in the current source 141B. The drain of the MOS transistor 15 isconnected to the power supply voltage source VDD and the gate thereof isconnected to the pixel output signal line VSL. In addition, the drain ofa MOS transistor 16, serving as a current source, is connected to thepixel output signal line VSL and the gate of the MOS transistor 15. Thesource of the MOS transistor 16 is connected to the reference potential(the ground potential GND in the present embodiment) and the gatethereof is connected to a bias-signal supply line VBIASL11.

The gates of the load MOS transistors LT1 to LT3 in the current source141B are connected to a bias-signal supply line VBIASL12.

The reason why the above-described structures are used is as follows.

Since each pixel output signal line VSL is generally arranged in thevertical direction (along the columns) of the pixel array 12, many unitpixels 11 are connected to each pixel output signal line VSL.

Accordingly, the pixel output signal line VSL has a parasiticcapacitance of several picofarads in some cases. When the drive currentamount of the load MOS transistors shown in FIG. 4, 6, or 7 is changed,the settling characteristics of a source follower output may bedeteriorated.

To prevent the deterioration, the output of the pixel output signal lineVSL is temporarily supplied to the source follower 15S and the drivecurrent amount of the load MOS transistors to the source follower 15S ischanged, thus achieving the above-described feedback.

The arrangement shown in FIG. 10 can achieve a ΔΣ analog-to-digitalconverter even in a solid-state imaging apparatus in which parasiticelements are connected to each pixel output signal line VSL.Advantageously, a solid-state imaging apparatus having column-parallelhigh-accuracy analog-to-digital converters can be achieved.

The imaging apparatus 10 having the above-described advantages can beused as a solid-state imaging apparatus included in a digital camera ora video camera.

FIG. 11 illustrates the structure of a camera system to which theimaging apparatus according to the present embodiment of the presentembodiment is applied.

Referring to FIG. 11, the camera system 300 includes an imaging device310, an optical system for allowing incident light to enter a pixel areaof the imaging device 310 (forming a subject image in the pixel area),e.g., a lens 320 for focusing incident light (image light) onto theimaging surface of the imaging device 310, a drive circuit (DRV) 330 fordriving the imaging device 310, and an image processor (PRC) 340 forprocessing an output signal of the imaging device 310.

The above-described imaging apparatus 10 can be used as the imagingdevice 310 in FIG. 11.

The drive circuit 330 includes a timing generator (not shown) forgenerating various timing signals, such as a start pulse and a clockpulse, used for driving a horizontal shift register and a vertical shiftregister and drives the imaging device 310 using predetermined timingsignals.

The present camera system 300 can obtain the same advantages as those inthe above-described embodiment. Advantageously, a high-resolutionhigh-accuracy camera capable of achieving high speed operation forrealizing a high frame rate or processing an increased number of pixelswithout causing an increase in power consumption and requiring animprovement of the accuracy of elements can be achieved.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A camera, comprising: an imaging apparatus; anoptical system that forms a subject image on the imaging apparatus,wherein the imaging apparatus includes: a pixel that outputs signalcharge as a pixel signal, the signal charge being obtained by imaging; acurrent source connected to a transmission path for the pixel signal,the current source having a variable current; and a signal processingcircuit that performs signal processing on a signal depending on anoutput signal to the transmission path and performs control so that acurrent of the current source is changed in accordance with the resultof signal processing, wherein, the signal processing circuit comprises:an integrator that integrates a pixel signal transmitted through thetransmission path; a quantizer that quantizes an output signal of theintegrator; and a selector that changes a current of the current sourcein accordance with the result of quantization by the quantizer; a drivecircuit; and an image processor.
 2. A camera, comprising: an imagingapparatus; an optical system that forms a subject image on the imagingapparatus, wherein the imaging apparatus includes: a pixel that outputssignal charge as a pixel signal, the signal charge being obtained byimaging; a current source connected to a transmission path for the pixelsignal, the current source having a variable current; and a signalprocessing circuit that performs signal processing on a signal dependingon an output signal to the transmission path and performs control sothat a current of the current source is changed in accordance with theresult of signal processing; a drive circuit; and an image processor. 3.The camera of claim 2, wherein, the pixel includes an amplifiertransistor constituting a source follower that outputs signal charge asa pixel signal to the transmission path, the current source includes aplurality of load transistors and a plurality of switches, the gates ofthe load transistors being biased using a predetermined bias signal, thesources of the load transistors being connected in parallel to apredetermined potential, each switch being connected between the drainof the corresponding load transistor and the transmission path, thesignal processing circuit selectively turns on the switches in thecurrent source in accordance with the result of processing on the pixelsignal transmitted through the transmission path, and the signalprocessing circuit comprises (a) an integrator that integrates a pixelsignal transmitted through the transmission path, (b) a quantizer thatquantizes an output signal of the integrator, and (c) a selector thatselectively turns on the switches in the current source in accordancewith the result of quantization by the quantizer.
 4. The camera of claim2, wherein, the pixel includes an amplifier transistor constituting asource follower that outputs signal charge as a pixel signal to thetransmission path, the current source includes a load transistor and aplurality of switches, the source of the load transistor being connectedto a predetermined potential, the switches being connected to the gateof the load transistor and being connected to different bias-signalsupply lines, respectively, the signal processing circuit selectivelyturns on the switches in the current source in accordance with theresult of processing on the pixel signal transmitted through thetransmission path, and the signal processing circuit comprises (a) anintegrator that integrates a pixel signal transmitted through thetransmission path, (b) a quantizer that quantizes an output signal ofthe integrator, and (c) a selector that selectively turns on theswitches in the current source in accordance with the result ofquantization by the quantizer.
 5. The camera of claim 2, wherein, thequantizer compares the level of an integrated signal obtained by theintegrator with a predetermined threshold level and outputs the resultof comparison to the selector, and the selector changes a current of thecurrent source so that when the level of the integrated signal is higherthan the threshold level, the level of the transmission path is lowered,and when the level of the integrated signal is lower than the thresholdlevel, the level of the transmission path is raised.
 6. The camera ofclaim 2, wherein, the pixel includes an amplifier transistorconstituting a source follower that outputs signal charge as a pixelsignal to the transmission path, the current source includes a pluralityof load transistors and a plurality of switches, the gates of the loadtransistors being biased using a predetermined bias signal, the sourcesof the load transistors being connected in parallel to a predeterminedpotential, each switch being connected between the drain of thecorresponding load transistor and the transmission path, the signalprocessing circuit selectively turns on the switches in the currentsource in accordance with the result of processing on the pixel signaltransmitted through the transmission path, the quantizer compares thelevel of an integrated signal obtained by the integrator with apredetermined threshold level and outputs the result of comparison tothe selector, and the selector selectively turns on the switches in thecurrent source so that when the level of the integrated signal is higherthan the threshold level, the level of the transmission path is lowered,and when the level of the integrated signal is lower than the thresholdlevel, the level of the transmission path is raised.
 7. The camera ofclaim 2, wherein, the pixel includes an amplifier transistorconstituting a source follower that outputs signal charge as a pixelsignal to the transmission path, the current source includes a loadtransistor and a plurality of switches, the source of the loadtransistor being connected to a predetermined potential, the switchesbeing connected to the gate of the load transistor and being connectedto different bias-signal supply lines, respectively, the signalprocessing circuit selectively turns on the switches in the currentsource in accordance with the result of processing on the pixel signaltransmitted through the transmission path, the quantizer compares thelevel of an integrated signal obtained by the integrator with apredetermined threshold level and outputs the result of comparison tothe selector, and the selector selectively turns on the switches in thecurrent source so that when the level of the integrated signal is higherthan the threshold level, the level of the transmission path is lowered,and when the level of the integrated signal is lower than the thresholdlevel, the level of the transmission path is raised.
 8. The camera ofclaim 2, wherein, the transmission path includes a pixel output signalline that is connected to the source of the amplifier transistor in thepixel and is connected to the current source.
 9. The camera of claim 2,wherein, the transmission path includes (a) a pixel output signal lineconnected to the source of the amplifier transistor in the pixel and (b)a source follower transistor, the gate of the source follower transistorbeing connected the pixel output signal line, the source thereof beingconnected to the current source.
 10. A camera, comprising: an imagingapparatus; an optical system that forms a subject image on the imagingapparatus, wherein the imaging apparatus includes: a pixel arrayincluding a plurality of pixels arranged in a matrix, each pixeloutputting signal charge as a pixel signal, the signal charge beingobtained by imaging; and a plurality of pixel signal processing unitsarranged for respective columns of the pixel array, wherein, each pixelsignal processing unit includes (a) a current source connected to atransmission path for the pixel signal, the current source having avariable current and (b) a signal processing circuit that performssignal processing on a signal depending on an output signal to thetransmission path and performs control so that a current of the currentsource is changed in accordance with the result of signal processing,and the signal processing circuit comprises (a) an integrator thatintegrates a pixel signal transmitted through the transmission path, (b)a quantizer that quantizes an output signal of the integrator and (c) aselector that changes a current of the current source in accordance withthe result of quantization by the quantizer; a drive circuit; and animage processor.
 11. The camera of claim 10, wherein: the quantizercompares the level of an integrated signal obtained by the integratorwith a predetermined threshold level and outputs the result ofcomparison to the selector, and the selector changes a current of thecurrent source so that when the level of the integrated signal is higherthan the threshold level, the level of the transmission path is lowered,and when the level of the integrated signal is lower than the thresholdlevel, the level of the transmission path is raised.
 12. The camera ofclaim 11, wherein the transmission path includes a pixel output signalline that is connected to the source of an amplifier transistor in thepixel and is connected to the current source.
 13. The camera of claim11, wherein the transmission path includes: a pixel output signal lineconnected to the source of an amplifier transistor in the pixel; and asource follower transistor, the gate of the source follower transistorbeing connected to the pixel output signal line, the source thereofbeing connected to the current source.